A comfortable mattress is crucial to providing high quality sleep. One way of making a mattress more comfortable is to provide multiple lateral zones of differing firmness that correspond to different areas of the body of the user of the mattress. Different portions of the user's body exert different pressures on the mattress. Thus, the lateral zones under the user's shoulders and hips are made of softer foam than the lateral zones under the user's torso and legs in order to allow the user's shoulders and hips to sink into the mattress and to allow the user's spine to remain straight.
A typical zoned foam mattress is made by gluing together lateral rectangular blocks of foam in which adjacent blocks have differing hardnesses. The indentation load deflection (ILD) is one measure of hardness defined in the ISO 2439 standard. The standard defines ILD as the force that is required to compress the foam to a specified percentage of its original thickness using a circular plate of fifty square inches. For example, the 25%-compression ILD is the most commonly used ILD and is the number of pounds required to achieve the 25% compression. ILD is also measured at 40% and 60% compression.
FIG. 1 (prior art) is a cut-away perspective view of a conventional zoned foam mattress 10. Mattress 10 includes an upper foam layer 11, a zoned foam layer 12, and a bottom foam layer 13. Upper foam layer 11 is made of visco-elastic polyurethane foam, otherwise known as memory foam. A person using mattress 10 lies directly on upper layer 11 through a thin quilted fiber padding 14 sewn to the mattress cover 15. Bottom foam layer 13 provides support for the other layers and is made of “high density” (HD) polyurethane foam. The term of art “high density” foam is somewhat of a misnomer because upper layer 11 of memory foam has a higher density than does the HD polyurethane foam. Typically, the HD foam used in mattresses has a density of between 1.5 to 2.5 pounds per cubic foot, whereas memory foam typically has a density between three and 5.5 pounds per cubic foot. Zoned foam layer 12 rests on bottom foam layer 13.
Zoned foam mattress 10 is a Queen size mattress that is sixty inches wide and eighty inches from the top end 16 to the bottom end 17 of mattress 10. Zoned foam layer 12 includes longitudinally spaced, transversely extending lateral regions 18-21 of foam. A first lateral region 18 is located at top end 16 of mattress 10. A second lateral region 19 is disposed between first lateral region 18 and a third lateral region 20. The user of mattress 10 sleeps with his or her shoulders over second lateral region 19 and his or her hips above a fourth lateral region 21. Regions 19 and 21 have a lower ILD than do regions 18 and 20. Consequently, the user's shoulders and hips sink deeper into regions 19 and 21.
However, forming a mattress by gluing together lateral blocks of foam having different degrees of hardness complicates the manufacturing process and adds to the cost of the mattress. In addition, the many glued joints create more places for the mattress to come apart. A method is sought for making a zoned foam mattress that does not require gluing together lateral rectangular blocks of foam to form the zones of different hardness.